Zhiyao Zhang

Mechanically-induced π-shifted long-period fiber gratings.

A band-pass filter based on mechanically-induced multi-π-shifted long-period fiber gratings is proposed. The pass band width of the filter depends on the number N of the sub-gratings divided by π-shifts in the long-period fiber grating. The depth of the two lateral rejection bands can be changed by the amount of pressure applied to the fiber. This paper demonstrates a multi-π-shifted long-period fiber grating created by pressing a fiber between two periodically grooved plates. For N = 7 and LP12 mode coupling, the extinction ratio is 22.22 dB, and the pass band loss is 0.85 dB. For LP12 mode coupling, the pass band width varies from 14.23 nm to 39.31 nm when N increases from 2 to 10.

Broadband linearization in photonic time-stretch analog-to-digital converters employing an asymmetrical dual-parallel Mach-Zehnder modulator and a balanced detector

A broadband linearization scheme for time-stretch analog-to-digital converters (TS-ADCs) is proposed based on an asymmetrical dual-parallel Mach-Zehnder modulator and a balanced detector. The theoretical and simulation results indicate that, compared with the differential and arcsine operation method generally employed in TS-ADCs, the proposed scheme has a superior performance on enhancing the spur-free dynamic range and suppressing the even-order distortions and the third-order spurs even under a large modulation depth. Additionally, the proposed scheme realizes online linearization. Therefore, it has the potential to enhance the dynamic range of a broadband TS-ADC in real time.

Refractive Index Sensing by Using Mechanically Induced Long-Period Grating

In this paper, a refractive index sensor using a mechanically induced long-period grating is proposed and experimentally demonstrated. The sensing element is based on a composite optical waveguide, which is made of a single-mode fiber, a Teflon-cannula, and the medium pending to test. The research shows that the resonant wavelength of the high- order mode coupling is sensitive to the refractive index of the liquid. The pressure applied on the experimental setup should keep constant in the refractive index measurements for different liquids, where the mode coupling is in the undercoupling state. The sensitivity of the sensor for LP14 mode resonance is about 2.78 × 10-4 refractive index unit in the refractive index range from 1.33 to 1.43.

Calibration-free and bias-drift-free microwave characterization of dual-drive Mach-Zehnder modulators using heterodyne mixing

Abstract. An electrical method is proposed for the microwave characterization of dual-drive Mach–Zehnder modulators based on heterodyne mixing. The proposed method utilizes the heterodyne products between the two-tone modulated optical sidebands and frequency-shifted optical carrier, and achieves calibration-free and bias-drift-free microwave measurement of dual-drive Mach–Zehnder modulators with high resolution electrical-domain techniques. Our method avoids the extra calibration for the photodetector and reduces half the bandwidth requirement for the photodetector and the electrical spectrum analyzer through carefully choosing a half frequency relationship of the two-tone modulation. Moreover, our measurement avoids the bias drifting problem due to the insensitivity to the bias phase of the modulator under test. The frequency-dependent modulation depths and half-wave voltages are measured for a commercial dual-drive Mach–Zehnder modulator with our method, which agree well with the results obtained by the conventional optical spectrum analysis method.

All-Fiber Low-Pedestal Spectral Compression in a Novel Architecture Based on an SMF Cascading an HNLF-NOLM

A novel all-fiber low-pedestal spectral compression scheme is proposed and demonstrated. The scheme is based on an anomalously dispersive single-mode fiber (SMF) cascading a nonlinear optical loop mirror with a highly nonlinear fiber (HNLF) in the loop. Both numerical and experimental results show that the spectral pedestal after spectral compression in the HNLF can be efficiently suppressed by the nonlinear optical loop mirror through the chirp-related intensity filtering effect. The measured spectral pedestal energy ratio is 9.59% using the proposed scheme, which is nearly a quarter of that using the conventional alternative based on an anomalously dispersive SMF followed by a feedthrough HNLF.

Optical differentiator based on mechanically-induced long-period fiber-gratings

All-optical first-order temporal differentiator using a mechanically-induced long-period fiber grating (MI-LPFG) in a small diameter fiber is demonstrated. The coupling intensity of the grating can be enhanced by changing the pressure applied on the fiber. The experimental results show that the transfer function of the MI-LPFG in a small diameter fiber is a high precision first-order temporal differentiator.

Spectral compression in a comb-like distributed fiber and its application in 7-bit all-optical quantization

Abstract. Subpicosecond pulse spectral compression without the limitation of the adiabatic soliton transform status in a comb-like distributed fiber (CDF) is proposed and numerically demonstrated. The principle of spectral compression in a single concatenation of single-mode fiber and high nonlinear fiber is theoretically analyzed and numerically proved. A three-stage CDF is designed and simulated following the principle. The simulation results show that an overall spectral compression ratio of 56.23 is obtained. The designed three-stage CDF is used in an all-optical analog-to-digital system to improve the quantization resolution to 7.1 bit.

Second-Order Optical Differentiator Based on a Mechanically-Induced LPFG with a Single pi-Shift

An all-optical second-order temporal differentiator based on a mechanically-induced long-period fiber grating (MI-LPFG) with a single π-shift is demonstrated. The MI-LPFG is created by pressing a fiber between two periodically grooved plates with a π-shift located at the 3/4 length from the input end of LPFG. The coupling coefficient (κ) can be adjusted through changing the pressure applied on the fiber. The experimental results show that the transfer function of the proposed MI-LPFG can be adjusted to be close to the ideal transfer function of a second-order differentiator. The differential performance of the designed differentiator to a Gaussian pulse is also analyzed.

Limits to dispersion compensation in a linear Brillouin slow-light system employing SBS slow- and fast-light propagation in cascaded optical fibers

A theoretical model for quantifying the pulse distortion introduced by the stimulated-Brillouin-scattering (SBS)-induced (equivalent) dispersion in a linear Brillouin slow-light system is presented. Based on this model, a linear Brillouin slow-light system employing fast-light propagation for dispersion compensation is analyzed. The results show that the elimination of gain-nonuniformity-induced equivalent group-velocity dispersion can be achieved with the sacrifice of introducing much larger high-order (equivalent) dispersion effects. It is also shown that the simultaneous cancellation of gain-nonuniformity-induced equivalent group-velocity dispersion and third-order dispersion as presented in a recent article is impossible.

Bi-directional comb-fiber architecture for resolution improvement of optical quantization employing soliton self-frequency shift and spectral compression

We demonstrate a high-resolution quantization scheme based on soliton self-frequency shift (SSFS) and spectral compression in a bi-directional comb-fiber architecture. Our scheme is composed of a Sagnac-loop-based mirror and a comblike combination of two sections of interleaved single-mode fibers (SMFs) and high nonlinear fibers (HNLFs). The Sagnac-loop-based mirror is placed at the terminal of the bus line to reflect the optical pulses back to the bus line to achieve single-stage SSFS and three-stage spectral compression. Quantization with a resolution of 6.2-bit is obtained in the experiment, which is 1.2-bit higher than that of its uni-directional counterpart. Our bidirectional scheme can achieve higher quantization resolution with a small volume and a low cost compared with its uni-directional counterpart.